The present invention relates to a method for reading a radiographic image wherein an image-wise exposure is given by means of a radiation to a radiographic image-converting panel (hereinafter referred to as an image-converting panel) having a stimulable phosphor layer and then the surface of the image converting-panel is scanned by stimulating light to produce a stimulable emission and thereby an image information recorded is read.
When a stimulable phosphor is exposed to a radiation such as X-rays, ultraviolet rays, a part of their energy is stored in the stimulable phosphor corresponding to the amount of exposure and if such stored energy is irradiated by stimulating light, the stored energy is emitted as stimulable light emission corresponding to the exposure. Such phenomenon is used for recording images formed by various kinds of radiations. As an example Japanese Patent Publication Open to Public Inspection No. 12144/1980 (hereinafter referred to as Japanese Patent O.P.I. Publication) discloses a method wherein a image-converting panel having a stimulable phosphor layer is given an image-wise exposure by means of a radiation such as X-rays to absorb a radiation energy for forming a kind of latent image thereon, and then the surface of the image-converting panel is scanned by the beam of stimulating light and thereby the intensity of the stimulable emission emitted from a phosphor is read and thus a radiation image is obtained. In such a method, a laser beam is usually used as stimulating light and a photo-detector such as a photomultiplier is used for reading a stimulable emission and the luminous intensity of the stimulable emission is picked up as an electrical signal and, finally, the electrical signal is used as image reproduction means on a hard copy or on a CRT screen.
Aforesaid image recording method has advantages such as: a range of a radiation exposure to be recorded is extremely wide: an image information obtained as an electrical signal can freely be treated by a signal processing: and, an optimum image suitable for purposes may be obtained. However, this method has a problem to be solved. That is, the energy stored in stimulable phosphor is likely to decrease to a great extent with the lapse of time.
Due to aforesaid disadvantage, as shown in FIG. 2, the read intensity of stimulable emission decreases according to the prolongation of the time elapsed after the image-wise exposure (t.sub.0) by means of a radiation until scanning (t.sub.1) by means of a stimulating light, whereby accurate tone reproduction of the image becomes difficult.
As measures against aforesaid disadvantages. Japanese Patent O.P.I. Publication No. 67242/1983, for example, discloses a reading apparatus wherein a preliminary reading by means of a weak stimulating light is conducted in advance of the reading of an image information and the conditions for the formal reading and image-processing are established based on the output of aforesaid preliminary reading.
However, since the decay in the intensity of a stimulable emission with the lapse of time continues even for the period from the commencement (t.sub.1) of the reading to the termination (t.sub.2) thereof, it is impossible to prevent an occurrence of density difference (shading) caused by the difference of density between the leading edge and the trailing edge of an image area in spite of the compensation made for the correction of the decay between aforesaid t.sub.0 and t.sub.1.